The Nuer:
A Description of the Modes of Livelihood and Political Institutions of a Nilotic People

E.E. Evans-Pritchard

The Nuer is one of the best known of all ethnographic studies, and the
reason for this is clear: Evans-Pritchard gives a brilliant and
insightful picture of an interesting and unusual people. The Nuer were
a pastoral people living along the upper Nile, who had no laws or
leaders and were strongly individualistic, with social order
maintained by community values and a segmentary tribal and lineage
system.

Most people are likely to encounter this in introductory anthropology
courses, but I think everyone should read at least one ethnography, even
if they never study anthropology: The Nuer wouldn't be a bad choice.